The development of an organism starts with the formation of the zygote. This single cell divides and a multicellular embryo is formed by a series of specific events well ordered in space and time, leading to the differentiation of a normally patterned organism. We use the fly Drosophila melanogaster to analyze these events because we have access to modern cellular and molecular techniques as well as genetical tools. Two of our investigations deal with the initial control of pattern formation. The first (1) asks how early events in the embryo are controlled. Specifically we will test whether or not a critical amount of nuclear material influences the timing of nuclear movement. The second (2) asks about the effect of the timing of these events, by inducing nuclear migration too early or too late. We will also probe the significance of the cell surface and cell communication in the differentiation of a pattern by investigating the following: (3) the role of cell heritable vs. nonheritable (e.g. the cells environment) components in forming the patterns of an organism and (4) the extent to which differences in cell commitment (determination) are manifested on the cell's surface. We will test whether or not cells sort out according to differences in cell determination and directly measure, using a cell binding assay, their recognition specificity. We will also compare surface properties of cells with the same determination but from different developmental stages, to see whether or not they share similar recognition properties. Finally by using immunological probes, we plan to identify cell surface antigens important in cell recognition. The results obtained in these studies and their interpretation will have implications not only for understanding the development of Drosophila, but also for developmental biology in general.